Delete extra Gecko files before shallow flash.

Base on the `flash_gecko()` function of `flash.sh` from B2G project [1].

1. remove `/system/media` on device (it seems like we don't need this folder...)
2. remove the extra Gecko files on device
3. shallow flash Gecko files into device

[1]: https://github.com/mozilla-b2g/B2G/blob/master/flash.sh#L262-L267